The present invention relates to Cartesian control systems.
In a telecommunication system, particularly in RF applications, efficient methods of linearisation are called for in order to minimize intermodulation, one cause of which being the distortion generated by non-linearities in power amplifiers.
In wide band systems, there are two main methods for dealing with distortion of amplifiers; predistortion and feedforward. Predistortion means that the distortion introduced by the main amplifier is inversely modelled and applied at the input of the amplifier, thereby making the total, ideally, equal to a power independent constant.
The feedforward technique consists basically of two independent steps. The first step is to extract the distortion introduced by the main amplifier on the signals to be amplified. This is referred to as extracting an error signal. The second step is to inject this error signal in anti-phase and time-aligned at the output of the feedforward amplifier to thereby cancel out the distortion. The performance of the feedforward technique is dependent upon the ability to add rotated signal vectors correctly in anti-phase and equal amplitude.
U.S. Pat. No. 5,157,346 to Powell et. al. discloses a feedforward lineariser with a Cartesian control system. Cartesian control in this case refers to the combination of complex Cartesian correlation detection followed by linear Cartesian feedback control.
U.S. Pat. No. 5,134,718 to Gailus discloses a method and device for correcting an initial phase relationship in a linear transmitter between an input signal and an input feedback signal, opening the feedback loop (open loop) and then calculating the phase from special injected training signals. The method of opening the feedback loop inherently has the disadvantageous consequence of interrupting the normal operation of the feedback loop, thereby causing disturbances in the system. Such a method is to be considered as an obtrusive method because it interrupts the normal operation of the feedback loop.
WO94/05078 to Bergsten and Nystrxc3x6m discloses a method of correcting the phase relationship within a Cartesian feedback loop at intermediate frequency.
In a Cartesian control system, the phase between the feedback and the input signals change with temperature, component ageing and input power etc. If Cartesian feedback and input signals are not phase-aligned then stability can be compromised. It is thus desirable to track and adjust these phase changes to ensure stability.
Dependencies on temperature, component ageing and input power etc. can alternatively be characterized and system parameters modified to allow for better start-up operation. This can be considered as an obtrusive phase adjuster technique, which requires off-line calibration. Such interruptions to normal operation are inconvenient if not unacceptable to the operation of most systems e.g. feedforward linearisers. Additionally, such modifications will lower the potential bandwidth, or speed, of the control system. Further, these modifications do not change the fact that the system is conditionally stable, therefore instability can still occur during operation.
Accordingly, it is an object of the present invention to provide a non-obtrusive control means for a Cartesian control system, said control means providing unconditional stability with respect to misalignment in phase between the feedback and input signals.
A further object of the invention is to provide a non-obtrusive method and device for a Cartesian control system that is unconditionally stable without reducing bandwidth or accuracy in the system.
A further object of the invention is to provide a method and a device for keeping feedback signals and input signals phase-aligned within a Cartesian control system in general.
Another object of the present invention is to provide a method and a device for a feedforward lineariser for RF-applications that is unconditionally stable.
Another object of the invention is to provide a method and a device for a phase adjuster technique for use in a feedforward lineariser, which is non-obtrusive to normal operation, i.e. requiring no off-line calibration.
Another object of the present invention is to provide a method and a device for a Cartesian feedback transmitter that is unconditionally stable.
Another object of the invention is to provide a method and a device for a phase adjuster technique for use in a Cartesian feedback transmitter, which is non-obtrusive to normal operation, i.e. requiring no off-line calibration.
These objects are achieved by a method and a device having the characterizing features of the independent claims. Further features and improvements of the invention, are given in the dependent claims.
According to the invention, a Cartesian control system is provided having a linear Cartesian loop feedback control system comprising a phase rotator and a phase adjuster. With a method and a device according to the invention, a system is provided that is unconditionally stable with respect to non-phase alignment, regardless of input power changes, temperature, manufacturing spread, component ageing etc. No certain conditions need to be placed upon the control system to ensure stability, and the system is non-obtrusive and requires no off-line calibration. The inclusion of the phase rotator and phase adjustment techniques into the Cartesian control system makes this possible.
Because the feedback and input Cartesian signals are kept phase-aligned, the stability margin requirements can be reduced. If this is applied to a known system, a wider bandwidth, i.e. a faster control technique can be achieved with the given stability margin. Thus a system is provided that is unconditionally stable without reducing the bandwidth or accuracy of said system.